Jovian Synchrotron Radiation Research Articles

Investigating Solar Wind‐Driven Electric Field Influence on Long‐Term Dynamics of Jovian Synchrotron Radiation

AbstractSince the first discovery of Jovian Synchrotron Radiation (JSR) from Jovian radiation belt in late 1950s, origin of its time variations has been one of the main subjects of the Jovian radiation belt study. JSR is reported to be strongly correlated with solar wind ram pressure with a possible time lag, which remains an unsolved issue. In our study, the influence of dawn‐to‐dusk electric field modulated with solar wind ram pressure on JSR variations is investigated. Alongside the original diffusion coefficient (DLL(UV)), a new diffusion coefficient (DLL(Elec)) which is dependent on solar wind conditions is applied to a one‐dimensional lossy radial diffusion model, and we reproduce long‐term variations of JSR between 1971 and early 2018. For the specific choice of DLL(UV) = 3.0 × 10−10L3[s−1], the correlation coefficient is found to be 0.6 between the simulated JSR and the ground observation data prior to 2005, and the intermittent observation after 2005 supports our simulation as well. We suggest that while DLL(UV) is the primary mode of diffusion that determines the steady profile of electron population and JSR, DLL(Elec) serves as a secondary mode, which controls long‐term variations of JSR.

Relation between the short‐term variation of the Jovian radiation belt and thermosphere derived from radio and infrared observations

AbstractWe report the first comprehensive observations of Jovian synchrotron radiation (JSR) and H3+ emission from the Jovian thermosphere to investigate the generation process of short‐term (days to weeks) variations in the Jovian radiation belt. The observations were made by the Giant Metrewave Radio Telescope and NASA Infrared Telescope Facility during November 2011. The total flux density of JSR increased by approximately 5% between 6–9 November and 12–17 November, associated with the increased solar UV/EUV flux. From 7 to 14 November, a possible rise in the infrared H3+ emission was observed in the middle‐latitude region, corresponding to a temperature variation of approximately 10 K. These results are consistent with the scenario that the solar UV/EUV heating causes variations in the thermospheric temperature and JSR. Radio images along the equatorial region showed that the JSR intensity decreased inside 1.5 Jovian radii (RJ) and the peak position shifted outward. This implies that energetic electrons are attenuated by some internal loss process, despite the simultaneous increase in radial diffusion. A physical model for the radiation belt shows that such an internal loss process can explain the observed variation of brightness distribution. Typical loss time scale is longer than strong diffusion limit, which suggests the existence of some pitch angle diffusion process such as wave‐particle interaction. Thus, variations of the total JSR flux density and thermospheric temperature seem consistent with the scenario, and the brightness distribution of JSR can be explained by the increase in radial diffusion accompanied by internal loss processes.

Assessment of Jovian synchrotron radiation with a bi-modal electron population

An explicit formula is developed to explore the mechanism of the synchrotron radiation by using a bi-modal loss-cone distribution function. The variation of the distribution function along the field line is modeled in detail and the evaluation of the total power in the synchrotron radiation is presented. The variance of synchrotron radiation with latitude depends on the electron anisotropy; for low anisotropy, synchrotron radiation increases with latitude and reaches a maximum at the particle mirror points; for high anisotropy, it decreases with latitude and maximizes at the equator. A bi-modal population is therefore suggested to explain the radiation intensity which peaks both at the equator and at high latitude.

Atmospheric loss of energetic electrons in the Jovian synchrotron zone

For decades, ground-based radio observations of Jovian synchrotron radiation have shown emission originating predominantly from the equatorial region and from high-latitude regions (lobes) near L∼2.5. The observations show a longitudinally asymmetric gap between the emission peaks of the lobes and the atmosphere of Jupiter. One possible explanation for these gaps is the loss of electrons through collisions with atmospheric neutrals as the electrons bounce along magnetic field lines and drift longitudinally in the presence of asymmetric magnetic fields. To assess this hypothesis, we applied the recently developed O6 and VIP4 magnetic field models to calculate the trajectories of electrons as they drift longitudinally in Jupiter's magnetic field, and derive the sizes of their equatorial drift loss cones. We then identified the shells on which electrons would be lost due to collisions with the atmosphere. The calculated drift loss cone sizes could be applied in future to the modeling of electron distribution functions in this region and could also be applied to the study of Jovian auroral zone. This method also allowed us to compute the shell-splitting effects for these drifting electrons and we find the shell-splitting to be small (⩽0.05 R J). This justifies a recent modeling assumption that particles drift on the same shells in a three-dimensional distribution model of electrons. We also compared the computed gaps with the observed gaps, and found that the atmospheric loss mechanism alone is not able to sufficiently explain the observed gap asymmetry.

Variations of Jupiter's synchrotron radiation: a link with solar activity?

The jovian synchrotron radiation has been regularly monitored with the Nançay Radiotelescope (NRT) between April 1994 and June 1999 at 21.3, 18.0 and 9.1 cm wavelength in horizontal and vertical linear polarization. Our five years data set clearly reveals the presence of medium-term (months) and long-term (years) natural variations in addition to the variations caused by the impacts of comet Shoemaker-Levy 9 in July 1994. The spectral index of the nonthermal flux density varies between −0.7 and 0.2 and is strongly correlated with the 9.1 cm flux. The long-term variation of the radio flux measured at the three wavelengths grossly follows the solar activity: we perform a correlation study of several parameters (temperature, density, dynamic pressure, solar energy flux, etc.) with the jovian flux density. The best correlation with the proton temperature, flow speed, dynamic pressure, ion thermal pressure and energy flux occurs for a time lag of ∼245 days. However the dynamic pressure, ion thermal pressure and energy flux curves also suggest a possible correlation with a time lag between 600 and 700 days. Clearly, other factors than the solar wind exert a control on the radiation belts in the close environment of Jupiter (like whistler-mode waves and ionospheric induced electric fields) that contribute to medium-term variations of the radio emission.

High-latitude radiation and resonant enhancements of the Jovian synchrotron emissions

A model describing (1) short time scale increases in Jovian synchrotron radiation and (2) emissions from high Jovian latitudes is presented. In the present model resonant interaction between oblique whistler waves and relativistic electrons diffuses the energy and the pitch angle of electrons which bounce along the Jovian magnetic field. The resonant interaction violates the first two adiabatic invariants and the energy diffusion results in hardening of the electron spectrum and in intensification of the synchrotron radiation. This model complements the radial diffusion process which (a) violates the third adiabatic invariant of a seed population via interaction with low-frequency magnetospheric oscillations and (b) increases the flux of energetic electrons at low L-shells over long time scales by conserving the first adiabatic invariant. The high-latitude emissions indicate also acceleration of bouncing electrons and radiation in the region of the strongest magnetic field. The seed particles and whistler waves determine the form of the electron distribution tail. This tail enhances the high-latitude synchrotron radiation observed at Earth. The resulting emissivities of the nonthermal radio waves in the ordinary/extraordinary modes of propagation, the polarization and the intensity spectrum observed at Earth due to these high-energy populations are calculated.

Observation of short term variation of Jovian synchrotron radiation at a frequency of 2290MHz

The detailed observation of Jovian decimetric radiation (DIM) from relativistic electrons in the Jovian radiation belt has been carried out to seek the existence of its short term variation. In the observation, the terrestrial atmospheric extinction effect was corrected using the sky tipping method. Moreover, the back ground confusion which was the serious problem of the past observations was also evaluated. As the result, the DIM flux enhancement with a duration of a few days in November 1996 was confirmed. The data also showed the changes of the magnetic latitude beaming curves during the flux enhancement. These results showed that the relativistic electrons in Jovian radiation belt which has been thought to be stable has a short term variation.

Modifications of the synchrotron radiation belts of Jupiter: evidence for natural variations in addition to SL9 effects

Regular observations of Jupiter at 21.3, 18.0 and 9.1 cm have been carried out with the Nançay Radio Telescope between 1 April 1994 and 11 April 1996. In July 1994, the collision of comet Shoemaker-Levy 9 with Jupiter produced a large increase of the nonthermal flux density with a hardening of the spectrum at higher frequency. This flux increase was followed in August and September by an increase of the east-west size of the emission region, mostly in the vertical polarization. In April 1995, the radiation belts had not fully recovered their pre-encounter levels. In October 1995, a new surge of the emission occurred characterized by a hardening of the flux and a broadening of the east-west (E-W) size. Two years of monitoring of the jovian synchrotron radiation suggest that the artificial effects induced by the Jupiter/Shoemaker-Levy 9 collision are interwoven with natural variations of the radiation belts.

Time variations of the Jovian synchrotron radiation following the collisional impacts of comet Shoemaker-Levy 9—2. Flux enhancement induced by neutral atmospheric turbulence

The observed increase of the decimetric radio emission from Jupiter after the atmospheric impacts of comet Shoemaker-Levy 9 may be the result of enhanced radial diffusion driven by the neutral wind dynamo. A simple model is considered here which invokes an increase of the radial diffusion coefficient by a factor of about two in the magnetic flux shell within 5° of the impact site latitude. The explosion-related neutral wind flow should be of a transient nature such that a continuous increase in the synchrotron radiation could only be sustained as long as there was energy release in the Jovian atmosphere. After the comet impacts, the decimetric radio emission will return to its normal level within 40 days.

Time variations of the Jovian synchrotron radiation following the collisional impacts of comet Shoemaker-Levy 9

Abstract The collisional interaction of the dust cloud associated with the nuclei- fragment chain of Comet Shoemakep-Levy 9 with the Jovian ring complex might lead to an increase of the ring mass. We estimate that the ring mass increase as a result of surface bombardment of the two inner satellites, Adrastea and Metis, could be relatively small and the upper limit should be 2 × 10 11 −10 12 g which is near the lower limit of the mass of the current Jovian ring. An increase in the ring mass, if significant, could produce observable reduction in the synchrotron radiation from energetic electrons as a result of the enhancement in tbe charged particle absorption effect. The resulting time variations of the radio emissions should also be diagnostic of the radial diffusion process near Jupiter.

Search for effects of comet S-L 9 fragment impacts on low radio frequency emission from Jupiter

Decametric radio observations of Jupiter were made before, during, and after the impacts of the fragments of the comet S-L 9 with the planet, from the University of Florida Radio Observatory, the Maipu Radio Astronomy Observatory of the University of Chile, and the Owens Valley Radio Observatory of the California Institute of Technology. The decametric radiation was monitored at frequencies from 16.7 to 32 MHz. The minimum detectable flux densities were on the order of 30 kJy, except for that of the large 26.3 MHz array in Florida, which was about 1 kJy. There was no significant enhancement or suppression of the decametric L-burst or S-burst emission with respect to normal activity patterns that might be attributed to the fragment entries. However, a burst of left-hand elliptically polarized radiation having a considerably longer duration than an L-burst was observed almost simultaneously with the impact of the large fragment Q2, and another with right-hand elliptical polarization was observed simultaneously with Q1. We consider the possibility that these two bursts were emitted just above the local electron cyclotron frequencies from the southern and northern ends, respectively, of magnetic flux tubes that had been excited in some way by the proximity of fragments Q2 and Q1. In addition to the monitoring of the decametric radiation, a search was conducted for possible comet-enhanced Jovian synchrotron radiation at 45 MHz using a large dipole antenna array at the observatory in Chile. This frequency is above the cutoff of the decametric radiation, but is considerably below the lowest frequency at which the synchrotron emission has previously been detected. The minimum detectable flux density with the 45 MHz antenna was about 5 Jy. No synchrotron emission at all was found before, during, or after the entry of the comet fragments.

Long‐term changes in Jovian synchrotron radio emission: Intrinsic variations or effects of viewing geometry?

A long‐term decadal scale variation of Jovian decimetric radio emission has been reported that could indicate intrinsic time variability in the Jovian radiation belts and/or magnetic field. However, the strong beaming of the radiation by trapped relativistic electrons in the complex Jovian magnetic field can also result in apparent decimetric radiation changes due to the evolving viewing geometry of Jupiter during its 11.9‐year orbit. To investigate this problem, radial diffusion models of Jovian trapped electron fluxes at relevant energies are first constructed to simulate possible causes of intrinsic variations. Results indicate several inconsistencies with observations. For example, an increase in diffusion rate with other parameters held constant results in an inward displacement of the peak emission radial distance that is not observed. Second, effects of viewing geometry changes are examined. The possible importance of such effects is suggested by a correlation between the total decimetric radio flux and the Jovigraphic latitude of the Earth, DE, which varies between −3.3° and +3.3° during one Jovian orbital period. Because the Jovian central meridian longitudes where the magnetic latitude passes through zero during a given Jovian rotation change substantially with DE and since significant longitudinal asymmetries exist in both the volume emissivity and the latitudinal profile of the beam, the total intensity should be at least a partial function of DE. Thus variations of viewing geometry associated with Jupiter's orbital motion should be more fully accounted for if long‐term intrinsic variability of Jovian synchrotron radiation is to be unambiguously detected.

The magnetosphere of Jupiter as observed with Pioneer 10: 3. Jovian synchrotron radiation at 10.4 cm as deduced from observed electron fluxes

Synchrotron radiation at 10.4-cm wavelength between 2.9 and 5.0 RJ (Jovian radii) has been calculated from the electron fluxes observed by the Iowa Pioneer 10 detectors. This calculated emissivity (W/m³ Hz sr) exceeds the Beard and Luthey (1973) spatial resolution of the Berge (1966) interferometer measurements by about a factor of 2 at 3 RJ. The calculated emissivity is quite insensitive to the energy spectral index. It is only moderately sensitive to the equatorial angular distribution. The disagreement would be only about 30% if Beard and Luthey had used the Iowa angular distribution in their work. A factor of 2 would represent a genuine disagreement, but 30% would not exceed the combined uncertainties of our analysis and the Beard-Luthey analysis.

The Radiation Belt and Magnetosphere

Book on radiation belt and magnetosphere covering Van Allen belt, Jovian synchrotron radiation, particle trapping, etc